When is the optimal time for delivery?--Purely from the fetuses' perspective.

To determine when the extrauterine environment becomes safer than the intrauterine environment with respect to fetal (neonatal) life, we analyzed all 4,896,505 livebirths, all 21,222 stillbirths, and all 7,513 early neonatal deaths after 26 weeks gestation that were recorded between 1989 and 1992 in Japan. Although the risk of early neonatal death ( < 1 week of age) and of neonatal death ( < 4 weeks of age) greatly exceed the risk of stillbirth at 26 weeks of gestation, those risks declined sharply by 39 weeks of gestation, then increased, while the risk of stillbirth within 1 and 4 weeks exceeded the risks of early neonatal death and of neonatal death at and beyond gestational weeks 40 and 38, respectively, for a singleton pregnancy. A similar reversal occurred at 37 and 35 weeks of gestation for multiple pregnancy. Neonates born at 39 and 37 weeks of gestation for singleton and multiple pregnancies, respectively, had the best outcome. It is concluded that the chance of survival for fetuses reaching 40 and 37 weeks of gestation for singleton and multiple pregnancies, respectively, were higher in the extrauterine than the intrauterine environment in Japan today.